Recent discoveries in several laboratories have led to important changes in our concepts of how T cells recognize antigens. These have involved a novel class of antigens termed "superantigens", thought to be important in self tolerance and in the pathogenesis of certain diseases. The only members of this class for which structural information is available are certain staphylococcal exotoxins that are etiologic agents in food poisoning and toxic shock syndrome. Essential features of these molecules include stimulation of large numbers of T lymphocytes based on TCR V(beta) expression in a manner dependent upon presentation by class II molecules, but apparently not restricted by specific self-MHC alleles. We and others independently reported that specific cellular receptors for the SEs are class II MHC molecules. We believe it likely that this interaction is fundamental to the remarkable immunological properties of the super- antigens. The overall objectives of this proposal are to explore SE/Class II interactions at a molecular level and to define their functional consequences on cells of the immune system. To this end, we propose six specific aims: 1) We will employ biochemical and molecular genetic techniques to define the region(s) of enterotoxin molecules that mediate two distinct activities, binding to class II MHC molecules and interaction with TCR; 2) Using similar procedures we will define regions of class II MHC molecules that bind enterotoxins, and employing a variant of HLA-DR1 characterized in our laboratory we will investigate the role of class II molecules in SE-mediated T cell activation; 3) We will investigate a general strategy for purification of prokaryotic and eukaryotic superantigens based on dye-ligand chromatography; 4) We will extend a preliminary observation that enterotoxin A binds to dinucleotide cofactor NAD to explore possible enzymatic activities of SEA; 5) We will determine the consequences of enterotoxin binding to class II-MHC molecules on monocytes and mast cells in transmembrane intracellular signal transduction; 6) Finally, in collaborative studies we will undertake crystallization of purified SEA and resolution of its 3-dimensional structure. Together the experiments proposed will contribute to understanding of how superantigens in general and staphylococcal exotoxins in particular interact with MHC class II molecules, leading to deletion of immature T cells, stimulation of mature T cells and the development of diseases as diverse as food poisoning, toxic shock syndrome and autoimmunity.